If user interaction is followed by a request to the server and has heavy task to do, you might want to prevent the user from triggering this multiple times. It is not always possible to solve this on the client, because:

User can reload the page directly after the action, so request never happens.

User can switch to another page which has to display something depending on previous changes.

Blocking the screen is not a good option from the UI side of view, also it will not prevent user from page reload.

Module will save the date of the last attempt to execute the function into the session store of connect. On every attempt to execute it again, it will check the date and prevent execution more often than the wait time defined for the function.

fn is a function which execution will be debounced. It can get an error object passed as first argument. Don't ignore it! Better to use a function name to avoid potential conflicts with some other debounced function which has the same implementation.